Device and Method for Detecting Smoke by Joint Evaluation of Two Optical Backscatter Signals

ABSTRACT

A smoke detector, on a base element with a flat installation surface, features a first light transmitter attached to the installation surface for emission of a first illumination light, a first light receiver attached next to the first light transmitter for receiving a first measurement light which results from a backscattering of the first illumination light at a measurement object located in a first detection area. The smoke detector further features a second light transmitter attached to the installation surface for emission of a second object illumination light, a second light receiver attached next to the second light transmitter for receiving a second measurement light which results from a backscattering of the second illumination light at a measurement object located in a second detection area, and a data processing device, which is used for joint evaluation of a first output signal of the first light receiver and of a second output signal of a second light receiver. In addition a method for the detection of smoke using the described smoke detector is specified.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority, under 35 U.S.C. § 119, of European patent application EP 08 101 743, filed Feb. 19, 2008, and which is herewith incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention lies in the technical field of alarm signaling technology. The present invention relates, in particular, to a device or system that is based on the principle of optical scattered light measurement for the detection of smoke. The present invention further relates to a method based on the principle of optical scattered light measurement for the detection of smoke.

Optical or photoelectric smoke detectors usually operate in accordance with the scattered light process. Such detectors exploit the fact that clear air reflects practically no light. If however there are smoke particles in the air, at least a part of an infrared illumination light emitted by a light emitting diode is scattered at the smoke particles. A part of this scattered light then falls on a light-sensitive sensor which is not illuminated directly by the light beam. Without smoke particles in the air the illumination light cannot reach the light-sensitive sensor.

A smoke alarm is known from EP 1 039 426 A2, which features a housing and a light transmitter and a light receiver arranged within the housing. A smoke detection area defined by the spatial arrangement of light transmitter and light receiver is located outside the smoke alarm. In order to enable a creeping contamination of the smoke alarm to be detected, the light transmitter is assigned a control receiver, which is configured for detecting radiation emitted by the light transmitter. In addition a control transmitter assigned to the light receiver is provided, so that the sensitivity of the light receiver can be checked.

A fire detector that is based on the known scattered radiation principle is described in German published patent application DE 10 2004 001 699 A1 and its counterpart patent application publication US 2008/0258925 A1 and. The fire detector features a number of radiation transmitters and a number of radiation detectors, the beam paths of which define a number of spaced scatter volumes or detection areas. The detection areas are spaced from each other so that small measurement objects, such as insects for example, cannot move through a number of detection areas at the same time. In this way a reliable distinction can be made between a light scattered at a small measurement object and a fire, in which smoke distributed over all detection areas will be distinguished.

A fire alarm which can be fitted flush into a ceiling of a room to be monitored is described in international PCT publication WO 2005/051053 and its counterpart patent application publication US 2007/0040695 A1. The fire alarm features a radiation transmitter and a radiation receiver which are accommodated alongside one another on a detector insert. Between a cover cap which separates the smoke detector from the room to be monitored and the detector insert a film can be inserted of which the color can be matched to that of the room to be monitored. A suitable choice of color enables the smoke detector to be adapted to the room to be monitored so that the smoke detector is not noticed or is barely noticed by people in the room.

A fire detector and a method for detecting fire is described in U.S. Pat. No. 5,225,810 and its European counterpart EP 0 472 039 A2. The fire detector features a laser light source configured to transmit short laser pulses in an area to be monitored. The fire detector also features a light detector which is arranged next the laser light source and which is configured to detect smoke in the area to be monitored or laser light scattered back from other objects by around 180°. On the basis of the time difference between transmitted and received laser pulses the position of a backscatter object within the area to be monitored can be determined. By a suitable comparison with time differences obtained by reference measurements the type of smoke detected can also be recognized. In particular a distinction can be made between black smoke and white smoke.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a device and a method for detecting smoke, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which, with little outlay on equipment, improves the detection of smoke, especially in respect of the likelihood of false alarms.

With the foregoing and other objects in view there is provided, in accordance with the invention, a device for detecting smoke, comprising:

-   -   a base element with a substantially flat installation surface;     -   a first light transmitter mounted to the installation surface         and configured to emit a first illumination light;     -   a first light receiver mounted to the installation surface         adjacent the first light transmitter and configured for         receiving a first measurement light resulting from a         backscattering of the first illumination light at a measurement         object located in a first detection area;     -   a second light transmitter mounted to the installation surface         and configured to emit a second illumination light;     -   a second light receiver mounted to the installation surface         adjacent the second light transmitter and configured for         receiving a second measurement light resulting from a         backscattering of the second illumination light at a measurement         object located in a second detection area; and     -   a data processing device connected to receive a first output         signal of the first light receiver and a second output signal of         the second light receiver, and configured to jointly evaluate         the first output signal of the first light receiver and the         second output signal of the second light receiver.

In accordance with a first aspect of the invention a device for detection of smoke is described. The described device has the following features (a) a basic element with a flat installation surface, (b) a first light transmitter which is attached to the installation surface and which is configured for transmitting a first illumination light and (c) a first light receiver which is attached to the installation surfaced next to the light transmitter and which is configured for receiving a first measuring light which results from a backscattering of the first illumination light at a measurement object located in a first detection area. The described device also features (d) a second light transmitter which is attached to the installation surface and which is configured for emitting a second illumination light (e) a second light receiver attached to the installation surface next to the second and light transmitter and which is configured for receiving a second measuring light which results from a backscattering of the second illumination light on a measurement object located in a second detection area, and (f) a data processing device which is coupled to a first light receiver and the second light receiver and which is configured for joint evaluation of a first output signal of the first light receiver and a second output signal of a second light receiver.

The described device for detection of smoke, which is subsequently also referred to in brief as a smoke detector, is based on the knowledge that the smoke detector can be implemented by a flat arrangement of all optoelectronic components on a common installation surface in an especially low-profile design. In such cases the first detection area and the second detection area are located outside the actual smoke alarm. The smoke alarm described thus involves an open smoke alarm.

It is pointed out that in practice smoke which is in the immediate vicinity of the smoke detector preferably contributes in a significant way to the measuring light received. Smoke which is further than around 104 mm away from the smoke detector is usually no longer registered by the latter since the corresponding optical backscatter signal is too small.

Against this background a distinction is made in this application between the two terms “detection area” and “field of vision of the smoke detector”.

The term “detection area” is to be understood as a layer which immediately adjoins the smoke detector. Smoke which is within the detection area will then lead to a significant and measurable optical backscatter signal. Smoke which is outside the detection area and which as a consequence is further away from the smoke detector will not contribute in a significant way to the received backscatter signal.

The term “field of vision of the smoke detector” is understood as the area which is basically covered by the smoke detector and which lies outside the detection area. As already explained above, smoke which is in the field of vision of the smoke detector cannot contribute in a significant manner to the optical backscatter signal. This does not apply however to concrete scattering objects such as insects and pieces of furniture for example. These can result in an appreciable optical backscatter signal even if they are only in the field of vision of the smoke detector.

Its flat profile makes it possible for the smoke detector described to be integrated without any great effort into the walls and especially into the ceilings of rooms to be monitored. Even if the described smoke detector is surface mounted is can easily be fitted to walls and/or ceilings. In such cases the smoke detector only occupies a small amount of space. In addition the described smoke detector can be accommodated such that it is not perceived as distracting by people in the room being monitored by the smoke detector or at least does not disrupt the design of the room.

The measurement object is especially smoke which comprises individual smoke particles which are detected by the described smoke detector based on the scattered light principle. The measurement object can however in practice also be another object such as an insect or an object accidentally brought into the detection area, which also generate a backscatter signal. The optical backscatter signals of actual objects located in the detection area such as especially insects are however significantly stronger by comparison with the optical backscatter signals caused by smoke. However a suitable evaluation of the output signals of the first and the second light detector by the data processing device enables such events to be reliably distinguished from the actual presence of smoke.

As a result of the backscatter geometry used, optics for the light transmitter and/or the light receiver are not necessary. This enables the described smoke alarm to be manufactured at especially low cost and to be suitable as a low-cost mass-produced product for monitoring in private rooms as well.

The scattered light is measured by the smoke detector described in a backscatter, geometry of around 180°. The deviation of the scatter angle from an exact backscatter and thereby from exactly 180° is produced (a) from the distance between the first and the second light transmitter and the first or the second light receiver and (b) from a distance between the location of the backscatter and the respective light transmitter or light receiver. For a scattering at a scatter object located in the detection area, as a result of a small layer thickness of the detection area, a sharp deviation from the scatter angle of 180° can be produced.

The described smoke detector differs especially in the backscatter geometry it uses from conventional smoke detectors which either, as forwards scatterers, have a scatter angle of approximately 60° or as backscatterers, have a scatter angle of around 120° between illumination light and scattered light.

The optoelectronic or photoelectronic components can advantageously be semiconductor diodes in surface mount technology. In this case the basic element can be a circuit board or at least feature a circuit board on which the semiconductor transmit and semiconductor receive diodes are accommodated in the known way and electrically contacted.

It is pointed out that within the framework of this application the term light basically means electromagnetic waves in any spectral ranges. This also includes the ultraviolet, the visible and the infrared spectrum for example. Light with the longer wavelengths such as microwaves for example also represent light within the meaning of this application. The term of light especially means electromagnetic radiation in the near infrared spectrum in which light-emitting diodes used as a light transmitter have an especially strong luminous intensity. The described smoke detector can however not only be used with almost monochromatic light radiation but also with light radiation which comprises two or more discrete wavelengths and/or a wavelength continuum.

In accordance with a further exemplary embodiment of the invention the first light transmitter and the first light receiver are implemented by a first reflection light barrier and/or the second light transmitter and the second light receiver by a second reflection light barrier. This has the advantage of enabling commercially available reflection light barriers to be used. No relative adjustment between a light transmitter and the corresponding light receiver for matching the direction of radiation of the light transmitter to the direction of reception of the light receiver is required as a result of the fixed relative arrangement of these optoelectronic components within a common component or at least within a common housing. The smoke detector can thus be constructed in an advantageous manner with a small installation outlay.

In accordance with a further exemplary embodiment of the invention the direction of the first illumination light in relation to a normal of the installation surface is inclined in the direction of the first light receiver and/or the direction of the second illumination light is inclined in relation to the normal of the installation surface is inclined in the direction of the second light receiver. In this context the term direction means the mean direction of radiation of the first and/or of the second light transmitter. This means that the light transmitters can not just be lasers such as for example a VCSEL (Vertical Cavity Surface Emitting Laser) which emit an almost parallel light bundle. The light transmitters can also have an emission characteristic with diverging light beams which have a certain angular distribution around the mean direction of emission inclined to the respective light receiver.

In accordance with a further exemplary embodiment of the invention the direction of the first illumination light and the direction of the second illumination light run in parallel to each other.

This can produce two detection areas separated spatially from each other, the spacing of which depends on the distance between the two light transmitters on the installation surface of the basic element.

In the case of diverging or spread-out light beams for the first and/or for the second illumination light, the direction of the illumination light relates respectively to the mean radiation direction.

In accordance with a further exemplary embodiment of the invention the first light transmitter and the second light transmitter as well as the first light receiver and the second light receiver respectively represent a limit of the device for detection of smoke. This means that neither the light transmitter nor the light receiver are located within a housing of the described smoke detector. Thus no other parts of the described smoke detector are located outside the photoelectric components light transmitter and light receiver. This also applies to covers or housing parts. The smoke detector can thus be embodied so that no further—possibly optically transparent—cover is located between the photoelectric components and the respective detection areas by which the photoelectric components are protected from contamination. These types of cover or contamination shields are however not even required for many applications, especially in the domestic area.

Furthermore the first detection area and/or the second detection area can also be located outside the smoke detector. In this case the device for smoke detection represents an open smoke detector which has no optical chamber of its own.

In accordance with a further exemplary embodiment of the invention the smoke detector additionally features a subtraction unit which on its input side is coupled to the first light receiver and the second light receiver and which on its output side is coupled to the data processing device.

The subtraction unit can for example be implemented by means of suitable hardware components which determine on the basis of analog output signals of the two light receivers a difference signal between the first output signal and the second output signal. This difference signal can then be evaluated in a suitable manner by a processor of the data processing device.

The subtraction unit can likewise be integrated into the data processing device and be implemented there either by means of hardware, by means of software or by means of a combination of hardware and software.

The influence of foreign light sources can for example be eliminated by the evaluation of the described difference signal, which radiates light from outside into the two light receivers and thus for each individual light receiver looks after an incorrectly increased receive signal.

In accordance with a further exemplary embodiment of the invention the first light receiver is configured for detection of a timing waveform of the first measurement light.

By detecting a backscatter light intensity detected by the first light receiver as a function of the time, information can be obtained in a simple manner about a scatter object intruding into the first detection area. If the intruding object is actually smoke, the change in the backscatter intensity over time occurs comparatively slowly, since smoke usually intrudes continuously into the detection area. By contrast if a concrete measurement object, such as an insect or a person for example, is accidentally brought into the field of vision of the described smoke detector, there will essentially be an abrupt change in the backscatter intensity. Thus, based on the strength of the change over time of the backscatter intensity detected in the first light receiver, reliable information can be obtained about the type of scatter object.

In accordance with a further exemplary embodiment of the invention the first light transmitter is configured to emit a pulsed first illumination light.

The use of pulsed illumination light with very short light pulses with a temporal length of preferably less than 1 ns in connection with a light receiver which has a timing resolution which is likewise in the nanosecond range has the advantage of information being able to be obtained about the spatial distribution of the light scatterers. In such cases for example a first optical reflection signal, which originates from the floor of a smoke detector arranged on the ceiling of a room, can be discriminated in time from a second reflection signal, which originates from being scattered on smoke. In this case the fact is exploited that smoke only delivers a significant reflection signal if is located within a detection area close to the smoke detector. Then a backscatter light assigned to the smoke can be assigned an insignificant light delay time. By contrast, for the backscatter light which originates from the floor of the monitored room, a finite light delay times defined by the distance between the smoke detector and the floor is measured. This exploits the fact that the light path predetermined by the light speed which the light pulse covers within a nanosecond is 30 cm long.

It should be understood in this context that the detection area can be a layer which is located immediately below a smoke detector arranged on the ceiling of a room. The layer thickness of the detection area can for example amount to around 10 mm for example.

Light scattered on smoke consequently has a signal path of approximately 5 to 20 mm. This produces a signal delay of 17 to 67 picoseconds. In other words, the smoke signal only has an insignificant and not measurable propagation delay and likewise a signal broadening which cannot be measured with sensible outlay. However actual scatter objects further away which might cause problems can be distinguished via the light travel time with a sufficient light resolution of the corresponding light receiver from a backscatter on smoke located close to the smoke detector. As an example an object at 15 cm from the detector, apart from a very strong signal, also produces a measurable signal delay of a nanosecond. In addition a pulse broadening may also be produced if the object has a number of backscatter areas which are at different distances away from the smoke detector.

Thus for example it can be concluded from a pulse duration of the received measurement light which is longer than the pulse duration of the corresponding illumination light, that the illumination light is being backscattered on different objects which are at different distances from the first light transmitter or the first light receiver. This time of temporal broadening or structuring of the measurement light pulse caused by different objects is consequently a reliable sign that that the scatter object located in the detection area is not smoke but reflections from the floor or at other close objects.

By contrast smoke leads to a non-measurable pulse broadening. This also applies if only low-cost components are used for the smoke detector and not high-performance optical measuring instruments for measurement of the delay time in the range of pico or femtoseconds. This means that, based on pulse length and pulse structure of the received measurement light signals, material objects such as insects or objects inadvertently brought into the field of vision of the alarm can be reliably differentiated from smoke which is in the detection area.

In addition a measurement of the time difference t between the sending out of an illumination light pulse and the backscattered measurement pulse detected by the light receiver enables it to be determined how far away the respective object is from the light transmitter or the light receiver. The space s between the object and the light transmitter or the light receiver is given by the following equation:

s=c ·t/2

The letter c stands for the speed of light here.

It will be understood that the second light transmitter can of course also be configured for emitting pulsed illumination light. Likewise the second light receiver can be configured for detecting a temporal waveform of the second measurement light.

In accordance with a further exemplary embodiment of the invention the smoke detection device additionally features a control device which is coupled to the first light transmitter and the second light transmitter and which is configured such that the first light transmitter is able to be activated independently of the second light transmitter.

An independent activation of the two light sources enables the described smoke detector to be operated in different operating modes. Thus for example an asymmetrical operating mode is possible, in which both the first light receiver and also the second light receiver is active, compared to a mode in which only one of the two light transmitters is switched on and the other is explicitly switched off. If in this operating mode both light receivers at least approximately show the same signal then a remote echo is involved. This can stem from a reflection of the illumination light emitted by the active light transmitter on an object far away, such as the floor of the room being monitored for example. In the event of danger, in which smoke intrudes into or occurs in the monitoring room, the smoke will also intrude into the near environment of the smoke detector, so that the two light receivers receive a greatly differing measuring signal. In this case the light receiver which is assigned to the switched-on light transmitter receives measurement light at a far greater intensity than the other light receiver.

In accordance with a further aspect of the invention a method for detecting smoke is described which uses the device described above. The smoke detection method described features (a) an emission of at least the first illumination light by the first light transmitter and (b) a reception of at least the first measurement light by the first light receiver, which results from a backscattering of the first illumination light at a measurement object located in a first detection area.

The idea underlying the method described is that measurement light received in backscatter geometry can create sufficiently strong signals to allow evaluation for a reliable detection of smoke. It is now possible from this knowledge, verified by the inventors in experimental trials, to implement smoke detectors within an especially compact design. In such cases the photoelectric components are able to be arranged on a common circuit board.

In accordance with an exemplary embodiment of the invention the method additionally features a reception of light be means of the second light receiver.

For an activation of just one of the two light transmitters of the smoke detector described above a common evaluation of the light intensities measured by the two light receivers enables smoke to be reliably detected. As presented above, a backscatter at spatially distributed smoke particles namely means that the light intensity received by the first light receiver will be far greater than the light intensity which relates to the second light receiver, which is assigned to the non-activated light transmitter. Only with light scattering at a very distant object will the two light intensities which respectively hit the two light receivers be at least approximately the same.

In accordance with a further exemplary embodiment of the invention the method additionally features a formation of a difference signal between the first output signal and the second output signal. The difference signal can be evaluated by a processor of the data processing device and any disruptive influence of the foreign light source can be eliminated in the process.

In accordance with a further exemplary embodiment of the invention the first illumination light features light pulses. The introduction of a time dependency for the illumination by the first light enables additional information about the spatial position of scatter objects located in the first detection area or in a field of vision assigned to the first light transmitter and the first light receiver to be obtained.

Naturally the second illumination light emitted by the second light transmitter can also feature light pulses.

In accordance with a further exemplary embodiment of the invention the method additionally features a measurement of the length of the light pulses scattered back as first measurement light. This has the advantage that information about the spatial arrangement of different objects within the field of vision of the smoke detector can be obtained.

Thus, depending on the pulse length, a large spatial distribution of the scatter objects leads to a temporal broadening or a structuring of the backscattered measurement light pulses by comparison with the emitted object illumination light pulses. The reason for this is that the illumination light is backscattered on various objects which are at different distances from the first light transmitter or of the first light receiver. Taking into account the finite speed of light, different optical wavelengths lead to a temporal broadening or structuring of the received illumination light pulses. By contrast, a backscattering at smoke does not lead to any clear lengthening of the measurement light pulse in relation to the illumination light pulse. As already explained above, the reason for this is that only smoke which is in the detection area in the immediate vicinity of the smoke detector delivers an appreciable backscatter signal. Smoke which is merely in the field of vision of the smoke detector usually delivers an optical backscatter signal which cannot be measured. On the basis of the pulse length and the pulse structure of the received measurement light signal, this enables physical objects such as for example insects or objects inadvertently brought in the field of vision of the detector to be reliably distinguished from smoke intruding into the detection area.

It should be understood that the temporal lengths of light pulses scattered back as second measurement light can also be measured and correspondingly evaluated.

In accordance with a further exemplary embodiment of the invention the method additionally features a measure of the time difference between the transmission of a measurement light pulse of the first illumination light and the reception of the corresponding measurement light pulse of the backscattered first measurement light. This has the advantage that the distance from a scatter object of the first light transmitter or the first light receiver can be determined in absolute terms.

Naturally the time difference between the emission of a measurement light pulse of the second object illumination light and the reception of the corresponding measurement light pulse from the backscattered second measurement light can also be measured.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as embodied in device and method for detection of smoke by joint evaluation of two optical backscatter signals, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL REVIEWS OF THE DRAWING

FIG. 1 is a schematic cross-sectional diagram of a smoke detector with two reflection light barriers attached to a common circuit board;

FIG. 2 is a diagram of a subtraction unit for forming a difference signal between two output signals of the light barriers shown in FIG. 1; and

FIG. 3 illustrates a temporal broadening or structuring of a measurement light pulse as a result of scattering at two objects at different distances from the smoke detector.

DETAILED DESCRIPTION OF THE INVENTION

It should also be noted at this point that the reference symbols of the same components or corresponding components in the drawings only differ in their first digit and/or through an appended letter.

Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown a smoke detector 100, which has a base plate 105. In accordance with the exemplary embodiment shown here the base plate is a circuit board 105 or a suitable circuit carrier for accepting electronic and optoelectronic components. All components fitted to the circuit board 105 are contacted in a manner not shown by means of conductor tracks or electrical wire connections in a suitable manner.

The smoke detector 100 comprises a first reflection light barrier 110 and a second reflection light barrier 120. The first reflection light barrier 110 features a first light transmitter 111 and arranged directly adjacent to it in a common housing, a first light receiver 112. The second reflection light barrier 120 features a second light transmitter 121 and arranged directly adjacent to it in a common housing, a second light receiver 122.

The first light transmitter 111 essentially emits a first illumination light 111 a perpendicular to the plane of the circuit board 105. The first illumination light 111 a is backscattered at least partly by approximately 180° in a first detection area 115, in which there is smoke for example. The backscattered light as first measurement light 112 a reaches the first light receiver 112.

In a corresponding way the second light transmitter 121 essentially emits a second illumination light 121 a perpendicular to the plane of the circuit board 105. The second illumination light 121 is backscattered at least partly by approximately 180° in a second detection area 125 in which there is smoke for example. The backscattered light, as the second measurement light 122 a, reaches the second light receiver 122.

The smoke detector 100 further features a subtraction unit 136 which forms a difference signal from the output signals of the two light receivers 112 and 122. This difference signal is fed to a data processing device 135 of the smoke detector 100.

A control device 130 is also provided which is coupled to the two light transmitters 111 and 121. This enables the two light transmitters 111 and 121 to be activated or switched on independently of each other.

All components 110, 120, 130, 135 and 136 of the smoke detector 100 are attached to the circuit board 105 and electrically contacted in a suitable manner. This enables the smoke detector 100 to be implemented in a very flat profile. The height of the smoke detector 100 in this case is determined merely by the thickness of circuit board 105 and by the components 110, 120, 130, 135 and 136.

In accordance with the exemplary embodiment shown here all components 110, 120, 130, 135 and 136 are Surface Mount Technology (SMD) components. This allows an overall height of just 2.1 mm to be achieved for example. The total height in this case is produced by the distance between the upper side of the circuit board 105 and the lower surface of the smoke detector labeled in FIG. 1 with the reference symbol 140.

In accordance with the exemplary embodiment shown here the light-active surfaces of the light transmitter 111, 121 and the light receiver 112, 122 coincide with the surface 140. This means that between these light-active surfaces and the respective detection areas 115, 125 there are no further parts of the smoke detector 100. This also applies to the covers or housing parts. These types of covers, which are frequently provided with known smoke detectors for the purposes of excluding contamination, are not required at all with many applications, especially in a household context, i.e., in the domestic area. In addition light barriers can also be used which have transparent protection layers for the light-active surfaces of the light transmitter 111, 121 and the light receiver 112, 122, so that at least a degree of contamination protection is provided by them.

The smoke detector 100 described with two reflection light barriers aligned in parallel has the advantage that it does not feature any optical elements such as for example lenses or mirrors. This enables the smoke detector to be manufactured in an especially simple manner with low-cost components. There are also no special installation tolerances to take into account in the assembly or installation of the smoke detector. All components required for the smoke detector are mass-produced components which can be manufactured at low cost.

It is pointed out that with the parallel light beams 111 a, 112 a, 121 a, 122 a there is principally the danger of distant solid objects being able to be interpreted as smoke. A concrete object accidentally moved into the vicinity of the smoke detector 100 in the detection area can be differentiated very well from a smoke signal on the basis of the very strong backscatter signal. Scattered objects further away which are located in the field of vision of the smoke detector, as a result of their mostly diffuse backscatter, only deliver a weak signal and are frequently no longer able to be distinguished with the above criterion reliably from smoke.

A reliable device for distinguishing between smoke and distant concrete scatter objects can however be effectively undertaken with the described smoke detector 100 in that for example, during the emission of the illumination light 110 a by the active light transmitter 111, the other light transmitters 121 are switched off or deactivated. Simultaneously the two light receivers 112 and 122 are activated. If in this case the two light receivers 112 and 122 show at least approximately the same signal, then a remote echo of an object which is located outside the detection area in the field of vision of the smoke detector is involved. This echo can for example originate from a floor surface of the area monitored by the smoke detector 100 and not from smoke particles. Smoke particles would namely, especially with ceiling mounting of the smoke detector 100, at least partly also be located in the vicinity of the smoke detector 100, so that in this case the signal of the two light receivers 112 and 122 would be of different strengths.

The difference signal between the two light receivers 112 and 122 can also be easily evaluated for detection of smoke. In such cases the influence of foreign light can also be effectively suppressed.

FIG. 2 shows the subtraction unit already shown in FIG. 1 which is now labeled with the reference symbol 236. A “plus input” of the subtraction unit 236 is fed with a first output signal 212 b of the first light receiver, which is labeled in FIG. 2 by the reference symbol 212. A “minus input” of the subtraction unit 236 is fed with a second output signal 222 b of the second light receiver, which is labeled in FIG. 2 with the reference symbol 222. A difference signal 236 b is formed from the two output signals 212 b and 222 b which is fed in FIG. 2 to a data processing device not shown. The difference signal 236 b can be evaluated in the data processing device as described above.

FIG. 3 illustrates in a schematic diagram the temporal broadening or structuring of a measurement light pulse as a result of the scattering at different spatially distributed objects 315 a and 315 b. The objects 315 a and 315 b expressly do not involve smoke. From the degree of temporal broadening or of temporal structuring conclusions can be drawn about spatial distribution of the solid scatter objects 315 a and 315 b.

As can be seen from FIG. 3, a light transmitter 311 transmits an illumination light 311 which features at least one short light pulse 313. This light pulse 313 is then scattered back at the objects 315 a and 315 b located in the field of vision of the detector by approximately 180 degrees. In this case the backscatter occurs at all possible objects within the field of vision of the detector 311, 312. For reasons of clarity however only two objects are taken into account on the illustration in FIG. 3. A typical object 315 b is located at a distance d from the light transmitter 311, the other typical object 315 a is located at a distance d′ from the light transmitter 311. In accordance with the exemplary embodiment shown here, the object 315 a involved is the floor of a monitored area. The object 315 b can involve any given object such as for example a piece of furniture, which is located permanently or temporarily between the floor 315 a and the smoke detector 311, 312.

The light pulse 313 initially hits the first object 315 b at a distance d from the light transmitter. In this case a part of the light energy is scattered back, so that the measurement light 312 a, which hits the light receiver 312 features a first backscatter pulse 313 a. Thereafter the now slightly weakened light pulse 313 hits the first object or the floor 315 a, which is located at a distance d′ from the light transmitter. Again a part of the light energy is also scattered back at the floor 315 a, so that the measurement light 312 a has a second backscatter pulse 313 b.

When all objects involved are taken into account, an overlaying of a plurality of individual backscatter pulses results, and the resulting overall backscatter pulse is broadened considerably by comparison with the input light pulse 313. Because of the absorption by any smoke located in the field of vision of the detector, the intensity of the later backscatter pulses in such cases can be reduced by comparison to the backscatter pulses arriving earlier at the light receiver 312. This produces an asymmetrical form or an asymmetrical temporal waveform of the overall backscatter pulse which is labeled with the reference number 314 in FIG. 3.

In this connection it is pointed out that any smoke which may be in the field of vision of the detector which is at a distance of typically more than a few centimeters of the smoke detector 311, 312 does not make any appreciable contribution to the received optical backscatter signal.

To detect the above-described effect of broadening or structuring of the backscatter signal, not only the amount or the strength of the backscattering, but also the time gradient of the corresponding pulsed backscatter signal can also be evaluated for intelligent smoke detection. As already indicated above, in such cases each input light pulse 313 can generate a plurality of backscatter pulses 313 a, 313 b, . . . , which originate from the spatially distributed objects 315 a, 315 b. The further away an object is, the more the incoming light beam 311 a is attenuated by scattering and absorption of if nec. by smoke particles contained in the air of the room. The same occurs with the backscattered measurement light 312 a. The echo of the objects further away however also arrives later at the light receiver 312. Thus, on the basis of the time gradient of the received light pulse, the spatial distribution or arrangement of the objects located in the field of vision of the smoke detector can be determined.

It is pointed out that the delay time of the measurement light pulse from emission to reception in the backscattered measurement light pulse 314 is measured and from it the distance between the light transmitter 311 or the light receiver 312 and the objects 315 a and 315 b can be computed.

It is further pointed out the embodiments described here merely represent a restricted selection of possible embodiment variants of the invention. It is thus possible to combine the features of individual embodiments with each other in a suitable manner so that, for the person skilled in the art, a plurality of different embodiments are to be viewed as obviously disclosed with the embodiment variants explicitly shown here. 

1. A device for detecting smoke, comprising: a base element with a substantially flat installation surface; a first light transmitter mounted to said installation surface and configured to emit a first illumination light; a first light receiver mounted to said installation surface adjacent said first light transmitter and configured for receiving a first measurement light resulting from a backscattering of the first illumination light at a measurement object located in a first detection area; a second light transmitter mounted to the installation surface and configured to emit a second illumination light; a second light receiver mounted to said installation surface adjacent said second light transmitter and configured for receiving a second measurement light resulting from a backscattering of the second illumination light at a measurement object located in a second detection area; and a data processing device connected to receive a first output signal of said first light receiver and a second output signal of said second light receiver, and configured to jointly evaluate the first output signal of the first light receiver and the second output signal of said second light receiver.
 2. The device according to claim 1, wherein at least one of the following is true: said first light transmitter and said first light receiver together form a first reflection light barrier; and said second light transmitter (121) and said second light receiver together form a second reflection light barrier (120).
 3. The device according to claim 1, wherein at least one of the following is true: a direction of said first illumination light is inclined relative to a surface normal of said installation surface in a direction towards said first light receiver; and a direction of said second illumination light is inclined relative to the surface normal of said installation surface in a direction towards said second light receiver.
 4. The device according to claim 1, wherein a beam direction of the first illumination light and a beam direction of the second illumination light run parallel to one another.
 5. The device according to claim 1, wherein said first light transmitter and said second light transmitter as well as said first light receiver and said second light receiver respectively form an outer boundary of the device.
 6. The device according to claim 1, which comprises a subtraction unit having an input connected to said first light receiver, an input connected to said second light receiver, and an output connected to said data processing device.
 7. The device according to claim 1, wherein said first light receiver is configured for detection of a time gradient of the first measurement light.
 8. The device according to claim 7, wherein said first light transmitter is configured to emit of a pulsed first illumination light.
 9. The device according to claim 1, which comprises a control device connected to said first light transmitter and to said second light transmitter and configured activate said first light transmitter independently of said second light transmitter.
 10. A method for detecting smoke, the method which comprises: providing a device according to claim 1; emitting at least the first illumination light with the first light transmitter; and receiving with the first light receiver the first measurement light which results from a backscattering of the first illumination light at a measurement object located in a first detection area.
 11. The method according to claim 10, which further comprises receiving light by way of the second light receiver.
 12. The method according to claim 11, which further comprises forming a difference signal between a first output signal of the first light receiver and a second output signal of the second light receiver.
 13. The method according to claim 10, which comprises generating the first illumination light featuring light pulses.
 14. The method according to claim 13, which further comprises measuring a length of light pulses backscattered as the first measurement light.
 15. The method according to claim 13, which further comprises measuring a time difference between an emission of a measurement light pulse of the first illumination light and a reception of a corresponding measurement light pulse of the backscattered first measurement light. 